The invention relates to a four wheel drive for a motor vehicle having a plate clutch arranged in between the front and rear axles, wherein inner clutch plates transfer increasing rotational moment to the outer clutch plates in dependence upon increasing rotational speed differences between the front and rear axles. A four wheel drive is described in German Published Unexamined Application (DE-OS) No. 21 35 791 in which a viscous lamella or plate clutch is built into the drive train. By means of different rotational speeds of the clutch plates, a rotational moment is effected via the shearing effect on the fluid in the clutch, which rotational moment is transferred to the axle (rear) which is not directly driven by the gear change transmission.
It is a disadvantage of the known art that the characteristic curve of the rotational moment with respect to the rotational speed difference has an upward but decreasing slope and not, as would be desired, an upward and increasing slope as does for example, the curve of a quadratic equation.
The most extreme loading of a viscous clutch of this type occurs in motor vehicles with front wheel drive when the directly driven front axle slips on very slick surfaces, for example, ice, and the rear axle is still on dry road. Under this condition, the front axle has a high rotational speed and the rear axle has a relatively low rotational speed, resulting in a large speed differential between the two axles.
Because a clutch of the type described above can transfer in the first gear the full motor load, that is, the full rotational moment, it is necessary to have a very steep characteristic curve depicting the moment transferred versus the axle speed differential. This steep slope, which represents an ability of the arrangement to transfer a large moment from the front axle to the rear axle at increasing speed differentials between the two axles, is necessary under driving conditions which create large speed differentials (such as occur in the above example) in order to avoid burning out the clutch.
If the characteristic line has too flat a slope, under conditions where the speed differential between the axles is very great, a "soft" viscous plate clutch results. In this case, the motor load is not transferred to the rear axle, resulting in a warming up of the clutch, which after a short time leads to an overheating and destruction of the clutch.
A steep characteristic curve at all speed differentials, however, is very disadvantageous as it corresponds to a reduction in precision steering. For example, driving into parking places is difficult because it is difficult to make sharp turns.
It is an objective of the invention to construct a four wheel drive with a plate clutch arrangement so that on the one hand, good driving handling is available for snow and ice, especially for driving around curves, and on the other hand, the full motor load is transferred to the not directly driven (rear) axle under certain hazardous driving conditions, at least for a limited time period.
To achieve this objective, a one way clutch and a reducing transmission are arranged to engage certain plates of the plate clutch at a predetermined axle speed differential. Where the plate clutch, which according to preferred embodiments is constructed as a viscous clutch having inner and outer plates, has a reducing transmission arranged in between it and the front axle, and wherein the plate clutch is connected to the reducing transmission by means of a one way clutch, a stepped rotational moment versus rotational speed differential characteristic curve results. The curve exhibits both a relatively flat upwardly sloping portion and a steeper upwardly sloping portion.
According to preferred embodiments of the invention, one group of inner clutch plates are rotationally fixed directly to the driving shaft and the other group of inner clutch plates are connected with the driving shaft via a one way clutch and a reducing transmission. The outer clutch plates are fixed to the clutch housing, which in turn, is connected to the rear axle. At relatively low speed differentials, only the inner clutch plates that are directly connected with the driving shaft are engaged to transfer the rotational moment to the outer clutch plates and, thus, the rear axle. At these rotational speed differentials the clutch operates along the relatively flat upwardly sloped portion of the characteristic curve.
Once the rotational speed difference reaches a certain predetermined value, however, the one way clutch engages to assist in the rotational moment transfer. Through the one way clutch, which is driven by the reducing transmission, the remaining inner clutch plates are engaged in the arrangement. When these remaining inner clutch plates are engaged, corresponding to an increasing rotational speed difference, the clutch functions in the steeply sloping portion of the characteristic curve. The predetermined switching point can be fixed by selection of a particular gear ratio and reducing gear transmission. In this manner, one can achieve a desired progressive overall characteristic line.
According to other preferred embodiments of the invention, all of the inner clutch plates are directly fixedly connected with the driving shaft from the front axle. In this embodiment the clutch exhibits a relatively flat upwardly sloping curve over a very broad spectrum of the rotational speed differences. After a predetermined rotational speed difference is reached, this curve extends vertically so that full slip differential transfer is achieved from the front axle to the rear axle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.